1. Field of the Invention
The invention relates to the field of bonding pads for magnetic bubble devices.
2. Prior Art
In the fabrication of single wall magnetic domain devices such as bubble memories, a plurality of layers are formed over the magnetic garnet layer. Typically a plurality of conductors are defined from a metal layer disposed over an insulative layer which insulates these conductors from the garnet layer. Permalloy elements are formed above this metal layer on another insulative layer such as a silicon dioxide layer. Then, to protect the permalloy elements, a scratch protection layer such as a glass layer is formed over the permalloy elements.
It is generally necessary to provide electrical contact to bonding pads formed from both the metal layer and permalloy layer. Ordinary masking and etching steps are employed to etch through the scratch protection layer and other insulative layers to expose these underlying bonding pads. Before actually bonding, however, these pads are subjected to the impact of probes for testing the plurality of devices formed on a wafer. This testing identifies acceptable devices before the bonding stage, to prevent the wasteful packaging and bonding of unacceptable devices.
The bonding pads, particularly those defined from the metal layer, are relatively thin, for example, 5,000 A thick. This thin layer is employed to provide a somewhat planar surface for the subsequent layers. The bonding pads defined from this metal layer are easily damaged by a probe tester and, to a lesser extent, by a bonding device. This problem, while not as severe, also occurs for the permalloy bonding pads.
Generally, two approaches have been taken to this problem. One approach is to accept the thin bonding pads and attempt to reduce the losses from the probe tester. A second approach is to build up the thickness of these bonding pads by depositing a metal layer such as a chromium gold layer on the scratch protection layer and exposed pads. This layer is then masked and etched to leave additional metal on only the bonding pads. While this latter approach provides thicker bonding pads, a reduction in yield results because of the additional masking and etching step.
As will be seen, the present invention provides a process for thickening these bonding pads without the additional masking and etching step required by prior art processes.